~Technical preparation.~ Fig. 81 represents a recent type of a carbide furnace. The base of the furnace is provided with a large block of carbon A, which serves as one of the electrodes. The other electrodes B, several in number, are arranged horizontally at some distance above this. A mixture of coal and lime is fed into the furnace through the trap top C, and in the lower part of the furnace this mixture becomes intensely heated, forming liquid carbide. This is drawn off through the taphole D. The carbon monoxide formed in the reaction escapes through the pipes E and is led back into the furnace. The pipes F supply air, so that the monoxide burns as it reenters the furnace and assists in heating the charge. The carbon dioxide so formed, together with the nitrogen entering as air, escape at G. An alternating current is used. [Illustration: Fig. 81] ~Calcium phosphate~ (Ca_{3}(PO_{4})_{2}). This important substance occurs abundantly in nature as a constituent of apatite (3Ca_{3}(PO_{4})_{2}.CaF_{2}), in phosphate rock, and as the chief mineral constituent of bones. Bone ash is therefore nearly pure calcium phosphate. It is a white powder, insoluble in water, although it readily dissolves in acids, being decomposed by them and converted into soluble acid phosphates, as explained in connection with the acids of phosphorus. STRONTIUM ~Occurrence.~ Strontium occurs sparingly in nature, usually as strontianite (SrCO_{3}) and as celestite (SrSO_{4}). Both minerals form beautiful colorless crystals, though celestite is sometimes colored a faint blue. Only a few of the compounds of strontium have any commercial applications. ~Strontium hydroxide~ (Sr(OH)_{2}.8H_{2}O). The method of preparation of strontium hydroxide is analogous to that of calcium hydroxide. The substance has the property of forming an insoluble compound with sugar, which can easily be separated again into its constituents. It is therefore sometimes used in the sugar refineries to extract sugar from impure mother liquors from which the sugar will not crystallize. ~Strontium nitrate~ (Sr(NO_{3})_{2}.4H_{2}O). This salt is prepared by treating the native carbonate with nitric acid. When ignited with combustible materials it imparts a brilliant crimson color to the flame, and because of this property it is used in the manufacture of red lights. BARIUM Barium is somewhat more abundant than strontium, occurring in nature largely as barytes, or heavy spar (BaSO_{4}), and witherite (BaCO_{3}). Like strontium, it closely resembles calcium both in the properties of the metal and in the compounds which it forms. ~Oxides of barium.~ Barium oxide (BaO) can be obtained by strongly heating the nitrate: Ba(NO_{3})_{2} = BaO + 2NO_{2} + O.